Geiger-mode focal plane array with monolithically integrated resistors

ABSTRACT

A GmAPD FPA having increased tolerance optical overstress includes a limit resistor that is monolithically integrated into each pixel in the FPA, and which limits the magnitude of the current entering the read out integrated circuit.

FIELD OF THE INVENTION

This invention relates generally to Geiger-mode (Gm) avalanchephotodiode (APD) focal plane arrays (FPAs) and, more particularly, toarchitectures and methods for increasing the optical overstresstolerance thereof.

BACKGROUND OF THE INVENTION

There is a potential for saturation or loss of functionality in GmAPDFPAs due to incoming high-intensity optical signals. Thesehigh-intensity signals can generate, in the GmAPD, excessively largecurrents, which are then injected into the read-out integrated circuit(ROIC). These large currents may result in corruption of data from theFPA or possibly result in damage to the sensor. It is desirable to avoidthese effects, collectively referred to as “optical overstress,” throughelements of the FPA design.

FIG. 1 depicts a circuit model for pixel 100 of a prior-art GmAPD FPA.In the context of a photodiode FPA, such as a GmAPD FPA, the term“pixel” collectively references (i) a single photodiode in the arraythereof and (ii) a unit cell of the read-out integrated circuit, at aminimum. Thus, pixel 100 includes GmAPD 102 (of an array thereofreferred to as a photodiode array or “PDA”) and ROIC unit cell 108.

The circuit model of the GmAPD 102 includes intrinsic instantaneousseries resistance “R_(S)(t),” breakdown voltage “V_(B),” diodecapacitance “CD,” and switch “S” to emulate the spontaneous avalanchebreakdown process in the GmAPD. See, Haitz, R. H. “Model for theElectrical Behavior of a Microplasma” J. Appl. Phys. 35, 1370 (1964).ROIC unit cell 108 includes arm transistor 110, disarm transistor 112,and sense transistor 114.

GmAPD 102 is armed by externally applied voltage “V_(HI),” withaccompanying low-impedance load resistance R_(L). Voltage V_(HI) is setas follows:V _(HI) =V _(B)+excess bias V _(EX)  (1)

where: V_(EX)<5 volts

The source of the excess bias is disconnected after a user-defined “arm”time.

Referring now to FIGS. 2A and 2B, detection of an optical signal, andits corresponding avalanche, at time t=t₀, corresponds to the closing ofswitch S. At this time, voltage V between nodes 104 and 106 in FIG. 1begins dropping from V_(HI) and current I begins increasing to I_(A):I _(A)=((V _(HI) −V _(B))/R _(S)(t))  (2)

When sense transistor 114 in ROIC unit cell 108 detects that the voltagehas dropped to quench voltage V_(Q), disarm transistor 112 turns “on,”reducing the voltage across GmAPD 102 to V_(LO), as depicted in FIG. 2A,where:V _(LO) =V _(HI)−5 volts  (3)

As the voltage is reduced below breakdown voltage V_(B), switch S opensagain and the current drops from steady-state value I₀ back to zero, asdepicted in FIG. 2B. At this point, GmAPD 102 can be re-armed with armtransistor 110 after a user-defined hold-off time. This approach, called“active quenching,” enables much greater control over the functionalityof a GmAPD in comparison with a “passive quenching” scheme, in which thedisarming and re-arming occur automatically in accordance with behaviorset by a single “quench” resistor in place of ROIC unit cell 108.

The initial spike in current I to I_(A) scales with optical intensity inthe presence of a very large optical signal due to R_(s)(t) decreasingwhen an excess of free carriers are generated within the diode. Highmagnitudes of this peak current ultimately cause the issues attributedto “optical overstress” upon injection into ROIC unit cell 108. As thisproblem is relatively unique to GmAPD FPAs due to the high impedancenecessary at the ROIC input for detection of avalanche events, to theinventors' knowledge, it has not been addressed in the prior art.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a way to significantly increase thetolerance of a GmAPD FPA to optical overstress.

A first approach adopted by the present inventors to address the problemof high peak current was to shut down the power supply lines in the ROICwhen high current is detected. This approach proved to be ineffective,however, due to the essentially instantaneous overstress impact of thecurrent pulse (i.e. <<10 nanoseconds). The inventors then considered asecond approach wherein the magnitude of the current is reduced beforethe current is injected into ROIC unit cell 108.

In accordance with the illustrative embodiment, one or more limitresistor(s) are monolithically integrated within each pixel of the GmAPDFPA. The inventors recognized that, based on certain considerationsrelated to the architecture of their GmAPD arrays, the limit resistor(s)must be monolithically integrated. In some of such embodiments, thelimit resistor(s) are integrated into the GmAPD of each pixel. In someother of such embodiments, the limit resistor(s) are integrated into theROIC unit cell of each pixel. In some further embodiments, limitresistor(s) are integrated into both the GmAPD and the ROIC unit cell ofeach pixel.

As a consequence of the presence of the limit resistor(s) in accordancewith the present teachings, when the switch S closes at time to, thevoltage V drops more slowly than in the prior art (as depicted in FIG.2A). Current still flows to the ROIC unit cell, but the total chargethat is injected therein is discharged at more gradually, therebyreducing the peak current magnitude as compared to the prior art (asdepicted in FIG. 2B). A further benefit of the embodiments of theinvention over the initially attempted “shut-down” approach is thathigh-intensity input signals can still be detected and analyzed.

Some embodiments of the invention provide a GmAPD FPA comprising aplurality of pixels, each pixel comprising: an electrical circuitincluding a GmAPD, a unit cell of a ROIC, and a limit resistor, whereinthe limit resistor is monolithically integrated in the pixel, andwherein the limit resistor limits a magnitude of a current entering theROIC unit cell, wherein the current is generated by the GmAPD.

Some embodiments of the invention provide a GmAPD FPA comprising aplurality of pixels, each pixel comprising: a GmAPD having a limitresistor monolithically integrated therein, the limit resistor operableto limit a magnitude of a current generated by the GmAPD, and a unitcell of a ROIC that receives the current, as limited by the limitresistor, from the GmAPD.

Some embodiments of the invention provide a method for increasing thetolerance of a GmAPD FPA to optical overstress, the method comprisingincreasing, within each pixel of the GmAPD FPA, the series resistance ofan electrical connection between the GmAPD and a unit cell of a ROIC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an electrical circuit model of a prior-art GmAPD FPApixel.

FIG. 2A depicts the voltage-vs-time behavior of the pixel of FIG. 1following an avalanche event.

FIG. 2B depicts the current-vs-time behavior of the pixel of FIG. 1following an avalanche event.

FIG. 3A depicts an electrical circuit model of a GmAPD FPA pixel inaccordance with a first embodiment of the present invention.

FIG. 3B depicts an electrical circuit model of a GmAPD FPA pixel inaccordance with a second embodiment of the present invention.

FIG. 4A depicts a comparison of the voltage-vs-time behavior of theGmAPD FPA pixel of FIG. 1 and a GmAPD FPA pixel in accordance with thepresent teachings, following an avalanche event.

FIG. 4B depicts a comparison of the current-vs-time behavior of theGmAPD FPA pixel of FIG. 1 and a GmAPD FPA pixel in accordance with thepresent teachings, following an avalanche event.

FIG. 5 depicts a flow diagram of method for fabricating a GmAPD PDApixel in accordance with the illustrative embodiment.

FIGS. 6A, 6C, and 6E depict cross-sectional views of a GmAPD PDA pixelin various stages of fabrication, in accordance with the illustrativeembodiment.

FIGS. 6B, 6D, and 6F depict top views of a GmAPD PDA pixel in variousstages of fabrication, in accordance with the illustrative embodiment.

FIG. 7 depicts a top view of a GmAPD FPA pixel at a stage offabrication, and showing an alternative layout of a limit resistor.

DETAILED DESCRIPTION

Embodiments of the invention provide a GmAPD FPA having increasedtolerance optical overstress. For the sake of clarity, the embodimentsof the invention are depicted and described at the pixel level. EachGmAPD FPA pixel includes a GmAPD pixel and a unit cell of an ROIC. Thereis a 1:1 relationship between GmAPDs and unit cells of the ROIC. Eachunit cell of the ROIC provides a digital readout of the avalanche eventsthat occur within the associated GmAPD. It is within the capabilities ofthose skilled in the art to appropriately scale to the level of a GmAPDFPA.

FIG. 3A depicts a circuit model of GmAPD FPA pixel 300A, in accordancewith an embodiment of the present invention. GmAPD FPA pixel 300Adiffers from prior-art GmAPD FPA pixel 100 in that the former includeslimit resistor 316A. In this embodiment, limit resistor 316A ismonolithically integrated into GmAPD 302 (as opposed to the ROIC unitcell). In an embodiment of a GmAPD FPA based on FIG. 3A, which willinclude a 2D array of pixels like that depicted in FIG. 3A, each suchpixel will include limit resistor 316A that has been monolithicallyintegrated within the GmAPD of the pixel.

In some embodiments in which limit resistor 316A is monolithicallyintegrated in GmAPD 302, the limit resistor is a thin-film resistor thatis patterned on the GmAPD from, for example, standard-production,high-resistivity NiCr and TaN thin films.

FIG. 3B depicts a circuit model of GmAPD FPA pixel 300B, in accordancewith an embodiment of the present invention. Like GmAPD FPA pixel 300A,GmAPD FPA pixel 300B differs from prior-art GmAPD FPA pixel 100 in thatpixel 300B includes limit resistor 316B. In this embodiment, however,limit resistor 316B is monolithically integrated into ROIC unit cell306.

Thus, in both embodiments depicted, the limit resistor is: (a) locatedin the circuit between the GmAPD and the ROIC unit cell, and (b)monolithically integrated into the GmAPD FPA pixel.

FIGS. 4A and 4B depict a comparison of the electrical behavior of aprior-art GmAPD FPA pixel (see FIG. 1) and a GmAPD FPA pixel inaccordance with the present teachings (e.g., see FIGS. 3A, 3B).

With reference to FIGS. 1, 3A, 3 b, and 4A, upon closing of switch S attime to, the voltage V in a GmAPD FPA pixel in accordance with thepresent invention (dashed line) drops more slowly than a GmAPD FPA pixelof the prior art (solid line). As depicted in FIG. 4B, current I stillflows to the ROIC, but the total charge injected into the ROIC (which isthe area under each curve in FIG. 4B) is discharged at a more gradualpace, thereby reducing the peak current magnitude as compared to theprior art.

Because sense transistor 114 in the ROIC unit cell is a voltagethreshold detector, the smaller current amplitudes generated inembodiments of the invention will not degrade the circuit's ability todetect avalanches. The primary performance trade-off to be considered isavoiding degradation of the FPA timing jitter performance associatedwith an increase in avalanche RC time constants induced by the presenceof limit resistor 316A or 316B. This places an upper limit on usefulvalues of the limit resistor of about 100 kOhms. The lower limit ofresistance of the limit resistor, which is about 1 kOhm, is determinedby the minimum value that reduces the peak current to an acceptablevalue.

Since the existing series resistance between the GmAPD anode and theROIC input in an APD FPA is dominated by the contact resistance of theAPD anode contact, which is about 100 ohms, the aforementionedresistance range of the limit resistor (i.e., about 1 kOhm to about 100kOhms) is expected to yield, at minimum, a factor-of-ten improvement inoptical-overload tolerance. At the same time, keeping the resistance inthe aforementioned range will, as previously noted, avoid unacceptablelevels of degradation in timing jitter performance.

As previously noted, in embodiments of the invention, the limit resistoris monolithically integrated into the GmAPD FPA pixel. A process formonolithically integrating the limit resistor into a GmAPD, such as toform GmAPD 301 (FIG. 3A), is disclosed in FIGS. 5 and 6A-6F and theaccompanying description.

The fabrication operations germane to embodiments of the invention takeplace after epitaxial growth of the various layers (e.g., absorptionlayer, charge control layer, cap layer, etc.) composing an APD(hereinafter referred to in the disclosure and claims as the “APD devicelayers”), but before diffusing a dopant into the cap layer to form theactive region of the APD. Conventional techniques are used for metaldeposition, insulator deposition, patterning, etc.

Referring now to FIG. 5, in operation S501 of method 500, a layerproviding electrical passivation (and also electrical insulation),referred to herein and the appended claims as a “passivation layer,” isdeposited on the APD device layers. The passivation layer comprises, forexample and without limitation, SiN_(x), Al₂O₃, and SiO₂. In operationS502, an opening is patterned in the passivation layer and dopant isdiffused through the opening, via standard techniques(photolithography/reactive ion etching, wet-etching, ion milling,sputtering, laser-assisted etching, etc.). It is within the capabilitiesof those skilled in the art to determine the size of the opening. If thematerial being doped is n-type semiconductor, the dopant will be ap-type dopant, such as zinc, cadmium, beryllium, and carbon. If thesemiconductor layer is a p-type material, then the dopant will be ann-type dopant, such as sulfur or silicon. It is within the capabilitiesof those skilled in the art to appropriately dope the cap layer toprovide an active region in the APD.

In operation S503, an n- or p-contact metal (dependent on device type)is deposited on a portion of the active region. As needed, additional“passivation” material is deposited to fill what remains of the openingformed in the passivation layer.

FIGS. 6A and 6B depict respective cross-sectional and top views of anascent GmAPD at the completion of operation S503. At this point in thefabrication, the GmAPD includes substrate 620, APD device layers 622,active region 624, passivation layer 626, and contact 628.

In operation S504, a layer of metal, such as, without limitation, highresistivity NiCr or TaN, is deposited between the contact (i.e., contact628) and a region in which a bond pad will be deposited. This layer ofmetal can be deposited, for example, via electron beam evaporation, orsputtering, and be patterned into a desired configuration usingtechniques known in the art. This layer of metal serves as the limitresistor.

In operation S505, additional metal is deposited in a location at whicha bond pad is desired. This metal will partially (or wholly) overlap themetal serving as the limit resistor. The bond pad comprises, for exampleand without limitation, gold, aluminum, copper, and alloys thereof.

FIGS. 6C and 6D depict respective cross-sectional and top views of thenascent GmAPD at the completion of operation S505. At this stage ofcompletion, the GmAPD includes limit resistor 630 and bond pad 632, inaddition to substrate 620, APD device layers 622, active region 624,passivation layer 626, and contact 628.

In operation S506, a layer of electrical insulation is deposited overthe APD. Materials suitable for use as the layer of electricalinsulation include, for example and without limitation, BCB, SiO₂,SiN_(X), and Al₂O₃. An opening is then formed around the bond pad toenable electrical contact between the GmAPD and the ROIC.

FIGS. 6E and 6F depict respective cross-sectional and top views of theGmAPD at the completion of operation S506, at which point the GmAPD, asmodified to include the limit resistor, is complete. The GmAPD includesinsulation layer 634, in addition to substrate 620, APD device layers622, active region 624, passivation layer 626, and contact 628, limitresistor 630 and bond pad 632. Also depicted is electrical connection636 from bond pad 632 to the ROIC.

FIG. 7 depicts resistor 730 having a serpentine shape, which is analternative embodiment of limit resistor 630. Limit resistor 730includes a greater number of resistor “squares” between the activeregion (i.e., active region 624) and bond pad 632, and thus presentsgreater electrical resistance. The excess of space available on thesurface of the PDA is what enables the incorporation of additionalresistor squares into the limit resistor. This approach, wherebyadditional resistor squares are added resulting in a serpentine (orother) shape, enables tuning the resistance of the limit resistor.

Monolithically integrating a limit resistor into an ROIC willnecessarily proceed along a somewhat different path as a consequence ofthe structural differences between a GmAPD and a ROIC. In light of thepresent teachings, those skilled in the art will be able to adapt ROICfabrication procedures to incorporate a limit resistor.

It is to be understood that the disclosure teaches just one example ofthe illustrative embodiment and that many variations of the inventioncan easily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

What is claimed:
 1. A Geiger mode avalanche photodiode (GmAPD) focalplane array (FPA) comprising a plurality of pixels, each pixelcomprising: an electrical circuit including: a GmAPD; a unit cell of aread-out integrated circuit (ROIC unit cell), wherein the ROIC unit cellcomprises an arm transistor, a disarm transistor, and a sensetransistor, wherein said transistors provide active-quenchingfunctionality for the GmAPD; and a limit resistor, wherein the limitresistor is electrically coupled to the GmAPD and the ROIC unit cell,and wherein the limit resistor is monolithically integrated in thepixel.
 2. The GmAPD FPA of claim 1 wherein the limit resistor ismonolithically integrated into the GmAPD of each pixel.
 3. The GmAPD FPAof claim 1 wherein the limit resistor is monolithically integrated intothe ROIC unit cell of each pixel.
 4. The GmAPD FPA of claim 1 whereinthe limit resistor is a thin-film resistor.
 5. The GmAPD FPA of claim 1wherein the limit resistor comprises materials selected from the groupconsisting of NiCr and TaN.
 6. The GmAPD FPA of claim 1 wherein thelimit resistor has a resistance in the range of about 1 kOhm to about100 kOhms.
 7. The GmAPD FPA of claim 1 wherein the limit resistor has aserpentine shape.
 8. The GmAPD FPA of claim 1 wherein the GmAPDcomprises: an active region formed within APD device layers; apassivation layer disposed on the APD device layers; an electricalcontact formed on a portion of the active region; the limit resistor,wherein the limit resistor is disposed on the passivation layer and isin electrical communication with the electrical contact; and a bond padformed on at least a portion of the limit resistor.
 9. A Geiger modeavalanche photodiode (GmAPD) focal plane array (FPA) comprising aplurality of pixels, each pixel comprising: a GmAPD having a limitresistor monolithically integrated therein, the limit resistor operableto limit a magnitude of a current generated by the GmAPD; and a unitcell of a read-out integrated circuit (ROIC unit cell), the ROIC unitcell having an active quenching circuit, wherein the ROIC unit iselectrically coupled to the limit resistor and receives the current fromthe GmAPD, as limited by the limit resistor.
 10. The GmAPD FPA of claim9 wherein the limit resistor has a resistance in the range of about 1kOhm to about 100 kOhms.
 11. The GmAPD FPA of claim 9 wherein the limitresistor is a thin-film resistor.
 12. The GmAPD FPA of claim 9 whereinthe limit resistor has a serpentine shape.